Capacitive discharge ignition system

ABSTRACT

An improved capacitor discharge ignition system for internal combustion engines. By regulating the peak primary current in a voltage step-up transformer for a DC-to-DC converter, a regulated quantity of energy is stored in the core of the transformer. The stored energy is used to charge a storage capacitor to a predetermined voltage. The charged capacitor is discharged through the primary winding of an ignition coil for generating a high voltage to fire a spark plug.

United States Patent 1191 Sieja Dec. 10, 1974 1 CAPACITIVE DISCHARGE IGNITION 3,406,672 10/1968 Phillips et a] 123/148 E SYSTEM 3,626,200 12/1971 Sasayama 123/148 E 3,677,255 7/1972 Withem 123/148 E [75] Inventor: Norman Francis Sieja, Toledo, Ohio [73] Assignee: Eltra Corporation, Toledo, Ohio Primary Examinercharle$ Myhre Assistant Examiner]oseph A. Cangelosi [22] F1led: Jan. 11, 1973 [21] Appl. No.: 322,789 [57] ABSTRACT An improved capacitor discharge ignition system for [52] U.S. Cl 123/148 E internal Combustion engines- By regulating the Peak [51] Int. Cl. F02p 3/06 primary current in a voltage stepup transformer for a 58 Field of Search 123/143 Dow-Dc Water, a regulated quantily of energy is stored in the core of the transformer. The stored en- [56] References Cited ergy is used to charge a storage capacitor to a prede- UNITED STATES PATENTS termined voltage. The charged capacitor is discharged e through the primary winding of an ignition coil for 3,372,681 3/1968 Ph ll ps et al .1 123/148 E generating a high voltage to fire a Spark plug 3,372,682 3/1968 Ph1ll1ps et a1... 3,372,683 3/1968 Phillips et a1 123/148 E 4 Claims, 1 Drawing Figure 11 14 15 g 69 if 35 25 32 CAPACITIVE DISCHARGE IGNITION SYSTEM BACKGROUND OF THE INVENTION This invention relates to electronic ignition systems and more particularly to an improved capacitive discharge ignition system in which a regulated quantity of energy is stored in the core of a transformer for charging a storage capacitor to a predetermined voltage.

Ignition systems for automobile engines and similar internal combustion engines are generally either of an inductive discharge type or of a capacitive discharge type. Capacitive discharge ignition systems usually outperform inductive discharge systems by providing a more uniformly high output voltage over a wide engine speed range. In a capacitive discharge ignition system, a battery is connected to charge a storage capacitor. An electronic switch such as a silicon controlled rectifier is triggered in synchronism with the engine to discharge the capacitor through the primary winding of an ignition coil, thereby establishing a high voltage across the ignition coil secondary winding for firing a spark plug. The electronic switch can be triggered by any conventional means, such as breaker points, a magnetic pickup or an optical pickup.

Capacitor discharge ignition systems are often provided with a DC-to-DC converter for charging the storage capacitor to a voltage considerably higher than the battery voltage. When used, the converters are sometimes designed to regulate the voltage to which the capacitor is charged. However, circuits of prior art DC- to-DC voltage converters used in capacitor discharge ignition systems typically have been one of the least reliable portions of the ignition system and, when such converters are provided with voltage regulating capabilities, they have been relatively expensive to build. When an ignition system is designed with regulating capabilities, it has the advantage of eliminating the need fora ballast resistor and a bypass switch to increase the voltage during starting or engine cranking when the battery is at low voltage due to the high cranking load.

SUMMARY OF THE INVENTION According to the present invention, a capacitive discharge ignition system is provided with an improved, highly reliable regulating DC-to-DC converter for periodically charging a storage capacitor to a substantially constant voltage above an applied battery voltage. The

converter includes a voltage step-up transformer for charging the storage capacitor to a voltage level several times higher than the relatively low battery voltage. Voltage regulation is achieved by limiting the current in the transformer primary winding to a predetermined maximum level, regardless of the value of the applied voltage within a predetem'iined voltage range. The transformer primary winding is connected across the battery in series with the input and output electrodes of a switching transistor and a current sensing resistor. The transistor is caus'edto conduct in response to each trigger pulse from a conventional pulse generator on the engine. As the current increases in the transformer primary winding, the current through the resistor is used as a control signal for limiting the maximum current passing through the transistor and, hence, through the primary winding. When the current ceases to increase, the transistor is switched to a nonconducting state and the collapsing magnetic field in the transformer core establishes a secondary voltage for charging the capacitor. By limiting the maximum current through the series connected transformer winding, transistor and current sensing resistor, the transistor is protected from possible damage due to either a high voltage from the battery or a high current caused by a short in the transformer primary winding. A circuit is also provided to protect the capacitor from accumulating a dangerously high charge in the event that the capacitor is not discharged.

Accordingly, it is a primary object of the invention to provide an improved capacitive discharge ignition system for internal combustion engines.

Another object of the invention is to provide an improved ignition system capable of operating over a wide range of input voltages.

Still another object of the invention is to provide an improved regulating DC-to-DC voltage converter for charging a storage capacitor in a capacitive discharge ignition system. I

Other objects and advantagesof the invention will become apparent from the following detailed description of a preferred embodiment, with reference being made to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows a schematic circuit diagram for a capacitive discharge ignition system constructed in accordance with the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to the single FIGURE, a capacitive discharge ignition system is shown embodying the principles of the present invention and is generally designed by the reference numeral 10. The ignition system 10 has a positive terminal 11 and a negative terminal 12 for connection to a suitable power source, such as a battery in a vehicle in which the ignition system 10 is used. The various components for the system 10 may be selected to provide a substantially constant output over a wide range of voltages, for example, from 6 volts to 35 volts. The ignition system 10 also has an input terminal 13 for receiving trigger pulses timed in synchronism with the operation of an internal combustion engine (not shown) with which the ignition system 10 is used. The trigger pulses applied on the input 13 may come from any conventional pulse source, such as breaker points driven by a cam on the engine, a magnetic pickup associated with either a disk with attached magnets or a toothed disk driven by the engine, or an optical pickup associated with a toothed or slotted disk driven by the engine. Pulse generating devices of these types are well known in the prior art andwill not be covered in detail herein.

The capacitive discharge ignition system 10 generally consists of a-DC-to-DC voltage converter 14 and a conventional capacitive discharge circuit 15. The capacitive discharge circuit 15 includes a storage capacitor 16, a switch such as a silicon controlled rectifier 17, and an ignition coil 18. The capacitor 16, the controlled rectifier l7 and the ignition coil 18 are connected to form a closed series circuit. When the controlled rectifier 17 is triggered, a charge on the capacitor 16 is discharged through the primary winding of the coil 18, thereby establishing a high voltage across the secondary winding of the coil 18 for firing a spark plug on the engine.

Triggering of the controlled rectifier 17 is achieved by a negative pulse applied on the input 13 and a circuit including a resistor 19, a capacitor 20 and a trigger transformer 21. The capacitor 20 is charged through the resistor 19. A negative pulse on the trigger input 13 discharges the capacitor 20 through the primary winding of the transformer 21 to trigger the silicon controlled rectifier 17 in synchronization with the internal combustion engine. A resistor 22 is connected as a load across the secondary of the transformer 21 to prevent multiple triggering of the controlled rectifier caused by the circuit oscillating.

When the controlled rectifier 17 is triggered, the charge on the capacitor 16 is discharged through the primary winding of the coil 18 to establish the high ignition voltage across the secondary winding of the coil 7 18. A diode 23 is connected in parallel with the primary winding of the ignition coil 18 to prevent energy stored in the core of the coil from establishing a reverse emf across the coil primary winding which would charge the capacitor 16. A resistor 24 is also connected in parallel with the diode 23 and a diode 25 is connected between the capacitor 16 and the negative terminal 12. The diode 25 and resistor 24 will also discharge any negative charge on the capacitor 16 and the diode 25 will protect the controlled rectifier 17 from a reverse volt age.

Each pulse on the trigger input 13 also activates the regulated DC-to-DC voltage converter 14 for recharging the capacitor 16 immediately after it is discharged to the ignition coil 18. A diode 26, the primary winding 27 of a voltage step-up transformer 28, a power transistor 29 or similar controlled conduction device and a current sensing resistor 30 are connected in series between the positive terminal 11 and the negative terminal 12. The secondary winding 31 of the transformer 28 is connected through a diode 32 to charge the capacitor 16. The transistor 29 is initially switched to a conducting state inresponse to a pulse on the trigger input 13 from the engine. Current then builds up in the primary winding 27 of the transformer 28 to a predetermined level to store a regulated quantity of energy in the core of the transformer 28, at which time the transistor 29 is turned off. The collapsing magnetic field in the transformer 28 establishes a predetermined high voltage across the secondary winding 31 for charging the capacitor 16 through the diode 32. The diode 26 merely serves as a safety device to prevent reverse currents from flowing through the series circuits if, for example, the polarity of a battery connected to the terminals 11 and 12 is accidentally reversed.

A transistor 33 and a transistor 34 are connected with the transistor 29 for forming a monostable switch. The emitter of the transistor 33 is connected to the positive terminal 11. Thebase of the transistor 33 is connected through a resistor 35 to the positive terminal 11 and through a resistor 36 to the collectors of the transistors 34 and 29. The collector of the transistor 33 is connected through a resistor 37 to the base of the transistor 34. The emitter of the transistor 34 is connected .for driving the base of the transistor 29 and also through a bias resistor 38 to the negative terminal 12. The transistors 34 and 29 are arranged in what is known as a Darlington connection with the transistor 34 driving the power transistor 29.

The trigger input 13 from the engine is connected through a capacitor 39 to the base of the transistor 33 for starting the converter 14. A negative pulse on the trigger input 13 drives the transistor 33 into saturation, in tum driving the driver transistor 34 and the power transistor 29 into conduction. Current will then increase in the series connected primary winding 27 of the transformer 28, the transistor 29 and the resistor 30 to a maximum level determined by the total resistance of the series circuit. As the current approaches a constant value, the voltage across the primary winding of the transformer 28 will approach a minimum level determined by the resistance of the primary winding of the transformer 28. This voltage is insufficient to maintain the transistor 33 in saturation. At this point, currents through the transistors 33, 34 and 29 decrease. With a decrease in current, a reversed emf is established across the primary winding of the transformer 28, causing the transistors 29 and 34 to stop conduction. A capacitor 40 is connected between the interconnected collectors of the transistors 29 and 34 and the baseof the transistor 34 to suppress a positive voltage transient at the collector of the transistor 29 when the transistor 29 switches off. The value of the capacitor 40 is chosen to improve the switching of the transistor 29 by reducing the maximum collector voltage without greatly increasing the switching time.

Regulation of the energy stored in the core of the transformer 28 is achieved by controlling the conduction of the transistor 29 when the current reaches a predetermined 'maximum level. A voltage will appear across the current sensing resistor 30 which is substantially proportional to the current flowing through the primary winding 27 and the transistor 29. A voltage divider consisting of a pair of resistors 41 and 42 is connected across the current sensing resistor 30. The com mon junction between the resistors 41 and 42 is connected to the control electrode or base of a transistor 43. The collector of the transistor 43 is connected to the base of the driver transistor 34 and the emitter is connected to the negative terminal 12 to shunt a portion of the current flowing through the resistor 37 to drive the base of the driver transistor 34. When the cur-- rent through the primary winding 27 of the transformer 28 approaches the-predetermined maximum level, the portion of the voltage across the current sensing resistor 30 which is applied by the resistors 41 and 42 to the base of the transistor 43 reaches a value equivalent to the base-emitter turn-on voltage of the transistor 43. At this point, the transistor 43 begins to conduct, shunting a part of the current to the base of the driver transistor 34 to the negative terminal 12. Through this arrangement, conduction by the transistor 29 is limited as the predetermined maximum permissible primary winding current is approached. At this point, the voltage across the primary winding 27 approaches a minimum and the transistor 33 switches the transistors 34 and 29 to nonconducting states. The magnetic energy stored within the core of the transformer 28 then collapses to establish a regulated high secondary voltage which charges the capacitor 16.

In some cases, it could be possible for the converter 14 to cycle even though the capacitor 16 has not been discharged by triggering the controlled rectifier 17. A voltage transient, for example, might be sufficient to cycle the converter 14 without triggering the controlled rectifier 17 or a faulty component could prevent discharging the capacitor 16. An over-voltage circuit is provided to protect the capacitor 16, the diode 32 and the controlled rectifier 17 against possible damage caused by over-charging the capacitor 16. As previously stated, one side of the capacitor 16 is connected through the resistor 24 to the negative terminal 12. The other side of the capacitor 16 is connected through a resistor 44 to the base of a transistor 45. A bias resistor 46 is also connected between the base and emitter of the transistor 45. The emitter of the transistor 45 is connected to the base of the driver transistor 34 and the collector is connected to the negative terminal 12 for shunting the base drive for the transistor 34 in the event that the voltage on the capacitor 16 exceeds a maximum permissible limit. The amount of base current available to the transistor 45 for over-voltage protection is dependent upon the ohmic value of the resistor 44 and the magnitude of the voltage on the storage capacitor 16. The value of the resistor 44 is a compromise. It must be large enough to keep the storage capacitor 16 from discharging too rapidly between converter cycles, yet it must be small enough to provide sufficient base drive for the transistor 45 when the supply voltage between the terminals 11 and 12 is at a maximum design value, for example, 35 volts on a system designed to regulate between 6 volts and 35 volts. At this time, the amount of base current available for the driver transistor 34 will be at its highest and, therefore, the drive requirement for the transistor 45' will also be at its highest. From this, it can be seen that the amount of voltage allowed to accumulate across the storage capacitor 16 when the controlled rectifier l7 fails to fire is dependent upon the supply voltage between the terminals ll and 12.

If the controlled rectifier 17 should fail to fire, the over-voltage protection circuit would at first shut the converter 14 completely off. Then, as the voltage across the storage capacitor 16 decays, a point will be reached where the transistor 45, though still conducting current, can no longer keep the converter 14 from cycling. Although the converter 14 can now be cycled, most of the base drive for the driver transistor 34 is still being shunted to the negative terminal by the transistor 45. The power transistor 29 is allowed to conduct only a'small amount of current through the primary winding 27 of the transformer 28. A small amount of voltage will appear across the secondary winding of the transformer 28 to increase the charge on the storage capacitor 16, again shuting down the converter 14. This onoff action will continue as long as the controlled rectifier 17 fails to fire or while for any other reason the capacitor 16 is not discharged. The voltage across the transistor 45 levels off at a value determined by the resistor 44 and the supply voltage between the terminals 11 and 12.

Accordingly, it will be apparent that the abovedescribed preferred embodiment of the capacitive discharge ignition system 10 includes a regulated converter 14 for charging a storage capacitor 16 to a predetermined level independent of the supply voltage between the input terminals 11 and 12 and also includes means for protecting the capacitive discharge circuit 15 from an over-voltage condition which could possibly damage components such as the capacitor 16 and the controlled rectifier l7. Furthermore, the system 10 will have a uniformly high output at engine cranking when design minimum without the use of a ballast resistor and a bypass switch. The regulating circuit which limits the current in the primary winding 27 will also protect the transistor 29 from high currents resulting from ei ther a short in the primary winding 27 or a high voltage across the terminals 11 and 12. It will be appreciated that various changes and modifications may be made in the above-described ignition system 10 without departing from the spirit and the scope of the claimed invention.

What 1 claim is:

1. A capacitive discharge ignition system for use with an internal combustion engine having a power source and means for producing timed pulses for triggering the ignition system comprising, in combination, a storage capacitor, a voltage step-up transformer having primary and secondary windings mounted on a core, a controlled conduction device having input, output and control electrodes, means connecting said primary winding, said input electrode and said output electrode in series, means for connecting said series connected transformer and controlled conduction device to the power source, means connected to said control electrode responsive to each trigger pulse for switching said controlled conduction device to a conducting state whereby current flows from the power source through said primary winding and said controlled conduction device to store energy in said transformer core, first means connected to said control electrode responsive to an increasing current flow through said series primary winding and controlled conduction device for increasing the impedance of said controlled conduction device to limit current flow through said controlled 7 conduction device to a predetermined maximum level whereby the total energy stored in said core is regulated, second means responsive to the voltage across said primary winding dropping to a predetermined minimum as the current through said primary winding increases to said predetermined maximum level for switching said controlled conduction device to a nonconducting state whereby a collapsing magnetic field in said core establishes a predetermined high voltage across said secondary winding, means for charging said capacitor with such secondary voltage, means responsive to each trigger pulse for discharging said capacitor, and means responsive to the discharge of said capacitor for generating a high voltage for firing a spark plug.

2. A capacitive discharge ignition system, as set forth in claim 1, wherein said means for limiting current flow through said controlled conduction device includes a resistor, means connecting said resistor in series with said primary winding and said input and output electrodes whereby a voltage is established across said resistor substantially proportional to the current flow through said primary winding, and means responsive to such voltage across said resistor reaching a predetermined level for limiting conduction of said controlled conduction device whereby the maximum current through said primary winding is regulated.

3. A capacitive discharge ignition system, as set forth in claim 2, and including means responsive to the voltage across said capacitor exceeding a predetermined level for inhibiting switching said controlled conduction device to a conducting state whereby the maximum charge on said capacitor is limited if said discharging means fails.

7 8 4. A capacitive discharge ignition system, as set forth tion device to a conducting state whereby the maxiin claim I, and including means responsive to the voltmum charge on said capacitor is limited if said disage across said capacitor exceeding a predetermined charging means fails.

level for inhibiting switching said controlled conduc- 

1. A capacitive discharge ignition system for use with an internal combustion engine having a power source and means for producing timed pulses for triggering the ignition system comprising, in combination, a storage capacitor, a voltage stepup transformer having primary and secondary windings mounted on a core, a controlled conduction device having input, output and control electrodes, means connecting said primary winding, said input electrode and said output electrode in series, means for connecting said series connected transformer and controlled conduction device to the power source, means connected to said control electrode responsive to each trigger pulse for switching said controlled conduction device to a conducting state whereby current flows from the power source through said primary winding and said controlled conduction device to store energy in said transformer core, first means connected to said control electrode responsive to an increasing current flow through said series primary winding and controlled conduction device for increasing the impedance of said controlled conductIon device to limit current flow through said controlled conduction device to a predetermined maximum level whereby the total energy stored in said core is regulated, second means responsive to the voltage across said primary winding dropping to a predetermined minimum as the current through said primary winding increases to said predetermined maximum level for switching said controlled conduction device to a nonconducting state whereby a collapsing magnetic field in said core establishes a predetermined high voltage across said secondary winding, means for charging said capacitor with such secondary voltage, means responsive to each trigger pulse for discharging said capacitor, and means responsive to the discharge of said capacitor for generating a high voltage for firing a spark plug.
 2. A capacitive discharge ignition system, as set forth in claim 1, wherein said means for limiting current flow through said controlled conduction device includes a resistor, means connecting said resistor in series with said primary winding and said input and output electrodes whereby a voltage is established across said resistor substantially proportional to the current flow through said primary winding, and means responsive to such voltage across said resistor reaching a predetermined level for limiting conduction of said controlled conduction device whereby the maximum current through said primary winding is regulated.
 3. A capacitive discharge ignition system, as set forth in claim 2, and including means responsive to the voltage across said capacitor exceeding a predetermined level for inhibiting switching said controlled conduction device to a conducting state whereby the maximum charge on said capacitor is limited if said discharging means fails.
 4. A capacitive discharge ignition system, as set forth in claim 1, and including means responsive to the voltage across said capacitor exceeding a predetermined level for inhibiting switching said controlled conduction device to a conducting state whereby the maximum charge on said capacitor is limited if said discharging means fails. 